The design of a 45 megawatt tetrode switch tube with water cooled grids, water cooled anode and matrix cathode is described. The choice of the matrix type cathode was made after consideration was given to all other popular types. Preliinary experments which led to the final assembly method are described. The calculation of the tube characteristics is explained in detail. Drawings and sketches of the various parts, subassembies and assemblies are included. A simplified proposed design is mentioned.